Detection of a Hot Subdwarf Companion to the Be Star FY Canis Majoris

Abstract

The rapid rotation of Be stars may be caused in some cases by past mass and angular momentum accretion in an interacting binary in which the mass donor is currently viewed as a small, hot subdwarf stripped of its outer envelope. Here we report on the spectroscopic detection of such a subdwarf in the Be binary system FY Canis Majoris from the analysis of data acquired by the IUE spacecraft and KPNO Coude Feed Telescope over the course of 16 and 21 years, respectively. We present a double-lined spectroscopic orbit for the binary based upon radial velocities from the IUE spectra and use the orbital solutions with a Doppler tomography algorithm to reconstruct the components' UV spectra. The subdwarf is hot (T_eff = 45+/-5 kK) and has a mass of about 1.3 M_sun and a radius of about 0.6 R_sun. It contributes about 4% as much flux as the Be star does in the FUV. We also present observations of the H-alpha and He I 6678 emission features that are formed in the circumstellar disk of the Be star. Orbital flux and velocity variations in the He I 6678 profile indicate that much of the emission forms along the disk rim facing the hot subdwarf where the disk is probably heated by the incident radiation from the subdwarf. A study of the FUV infall shell lines discovered in the 1980s confirms their episodic presence but reveals that they tend to be found around both quadrature phases, unlike the pattern in Algol binaries. Phase-dependent variations in the UV N V doublet suggest the presence of a N-enhanced wind from the subdwarf and a possible shock-interaction region between the stars where the subdwarf's wind collides with the disk of the Be star.Comment: 46 pages, 15 figures, ApJ in pres

A Spectroscopic Orbit for the Late-type Be Star β CMi

“…This star becomes the dominant light source and a Be star with a small, 1.5M ⊙ companion with a long orbital period of 126 d. This situation appears to be common for a wide variety of Be stars. Similar direct ultraviolet detections of hot subdwarf companions have been accomplished for FY CMa (Peters et al 2008), 59 Cyg (Peters et al 2013), HR 2142 (Peters et al 2016), and o Pup (Rivinius et al 2012, Koubský et al 2012) all with small mass functions in the range of 2 × 10 −3 − 1.3 × 10 −2 M ⊙ . Other Be binaries that have not had direct detections of the companion stars such as γ Cas (Smith et al 2012) or ζ Tau (Ruždjak et al 2009) also show similar mass functions (f (M ) = 1 × 10 −3 and f (M ) = 6 × 10 −3 respectively) that indicate low-mass companions.…”

“…The largest difficulty is obtaining high precision velocities with the rapid rotation, but this has been accomplished for the rapid rotator Regulus . Follow-up spectroscopy in the ultraviolet could easily show the spectroscopic signs of the companion, as evidenced by the results of Matson et al (2015), Peters et al (2008Peters et al ( , 2013Peters et al ( , 2016, or Gies et al (1998). Lastly, emission lines that form at very large radii should be investigated to look for further structure caused in the presence of a companion star.…”

“…We also attempted to use the techniques used previously for the Be stars known to have hot subdwarfs in their UV spectra (Gies et al 1998, Peters et al 2008. Unfortunately, only four spectra exist in the IUE archive for β CMi, which makes direct detection of the newly found companion unlikely for the given dataset.…”

The Use of Virtual Observatory Databases in Binary Star Research

“…More than twenty years ago researchers were beginning to suspect that some Be stars may have been spun up to their very rapid rotation rates through the process of mass transfer. It seemed impossible to confirm the presence Peters et al (2008) of a close hot subdwarf, that would eventually become a massive white dwarf, about a Be star because of the immense difference in their fluxes. But through the analysis of 16 IUE HIRES images of the bright Be star φ Per (B2 Vpe) with a Doppler tomography code (Bagnuolo et al 1994), Thaller et al (1995) were able to recover the spectrum of a hot sdO secondary with a T eff of ∼50 kK.…”

Summary of Observational Techniques

“…One example: the detection of a hot subdwarf companion to the Be star FY CMa (Peters et al 2008). With some amount of nostalgia, she recalled the International Ultraviolet Explorer as a "discovery machine" in the 1980s.…”